The present invention relates to a graded index plastics optical fiber and to a method of continuously fabricating a graded index plastics optical fiber.
Graded index plastics optical fibers which can be used in a band which preferably extends as far as the near infrared are beneficial because they can be used in broadband access networks. At present no such fibers are commercially available, although there has been increasing interest in research on this topic over some fifteen years. Various research approaches have been developed and the corresponding fibers have been synthesized experimentally. Fibers made of polymethacrylate cannot be used in the intended band because the presence of many Cxe2x80x94H bonds limits applications to the visible spectrum and to wavelengths shorter than 800 nm. Also, they cannot be used at temperatures of 80xc2x0 C. and above. One area that once seemed to be more promising is that of fibers essentially made of perfluorinated polymers, but as yet there is no commercially available material for this particular application. There are other research approaches of a hybrid nature.
For example, document WO-A-97/36196 describes a multilayer graded index plastics optical fiber (GIPOF) whose refractive index is higher at the center and decreases progressively towards the periphery, the fiber having a plurality of coaxial layers of (co)polymers selected from the group comprising homopolymers HP1, HP2, . . . , HPn of at least two monomers M1, M2, . . . , Mn and at least one copolymer CP of the monomers Mn, mixed layers of two similar (co)polymers being provided between each layer. The above patent describes two methods of preparing the GIPOF. One method consists in preparing materials for extrusion which are made up of the (co)polymers, extruding them using a coaxial multiple orifice nozzle and allowing the layers to diffuse into one another. That technique based on depositing previously melted polymers does not guarantee uniformly thick layers or reproducible interdiffusion. The other method entails preparing materials for extrusion which are made up of the (co)polymers and the monomers Mn, extruding them using a coaxial multiple orifice nozzle, allowing the layers to interdiffuse, and then photopolymerizing the monomers. For preparing the (co)polymers, that document also mentions that it is possible to part-polymerize them, for example up to at least 50% polymerization, and to continue the reaction by photopolymerization. Any such operation entailing stopping polymerization while it is in progress is essentially non-reproducible. Finally, the GIPOF obtained is made up of layers of resin consisting of interdiffused polymers of high molecular weight. There is no mention of any crosslinking by photopolymerization in that method. The polymers are therefore not chemically bonded, which leads to the refractive index profile being unstable in terms of duration and temperature.
Patent application JP-A-09 138 313 discloses a method of fabricating GIPOF by applying a solution obtained by diluting a polymer with a substance having a low refractive index in a monomer around a plastics core with the highest index. Polymerizing the monomer produces the first layer of polymer with a lower refractive index than the core. Various layers are deposited in the same manner, with the refractive index decreasing progressively from the center towards the periphery. In the example, the viscosity of the solutions applied to the core is in the range from 5000 poises to 10000 poises, i.e. 500 Pa.s to 1000 Pa.s. The substance with a low refractive index for use in the invention is not polymerizable. The refractive index profile is therefore unstable in terms of duration and temperature.
In the plastics optical fiber of the invention, the refractive index profile is stable in terms of duration and temperature. A fixed three-dimensional structure is created by crosslinking during fabrication of the fiber. Moreover, in the method of fabricating a plastics optical fiber of the invention, choosing the viscosity of the various mixtures enables them to be deposited coaxially and without melting. Consequently said method is highly reproducible, mainly in terms of interdiffusion and of the thickness of the layers, and the refractive index profile is therefore highly reproducible.
The invention also relates to two direct fiber fabrication processes which deposit specific mixtures of polymers whose refractive indeces vary as a function of fluorine content.
The present invention provides a graded index plastics optical fiber in which the refractive index is highest at the center and decreases progressively towards the periphery, said fiber having a substantially homogeneous and amorphous crosslinked structure made up of a material based on monomers and polymers that are at least partly fluorinated.
The invention also provides a method of continuously fabricating a graded index plastics optical fiber in which the refractive index is highest at the center and decreases progressively towards the periphery, said fiber having a plurality of layers of resins deposited concentrically, said method comprising
preparing at least two amorphous mixtures each comprising at least one primer, at least one at least partly fluorinated polymer P comprising from 10 to 1000 monomer units, and preferably from 20 to 100 monomer units, said polymer comprising a main chain and at least two reactive groups, generally in a lateral and/or terminal position of the main chain, and a reactive diluting agent comprising at least two monomers M1 and M2 with different refractive indeces, each monomer being at least partly fluorinated and including a reactive group adapted to react with a reactive group of the polymer P, the proportion as a percentage by weight of said polymer P relative to the components of the mixture being practically constant for each mixture and the proportion as a percentage by weight of M1 relative to the sum of monomers of the reactive diluting agent varying significantly from one mixture to another,
extruding the first mixture with the highest refractive index to constitute the core,
depositing layer(s) of mixture(s) having respective decreasing refractive indeces on said core,
interdiffusing each layer with the adjoining layer(s), and
crosslinking the polymer P and the monomers of the reactive diluting agent.
The crosslinking is preferably photocrosslinking and the primer is preferably a photoprimer.
In a variant of the method of the invention, the main chain of the polymer P is selected from the group comprising at least partly fluorinated poly(meth)acrylates which can optionally incorporate at least one at least partly fluorinated maleimide type monomer. In a second variant of the method of the invention, the main chain of the polymer P is selected from the group comprising at least partly fluorinated polyethers.
In an embodiment of the method of the invention the reactive groups of the polymer P are at least partly fluorinated polyethers.
In an embodiment of the method of the invention the reactive groups of the polymer P are at least partly fluorinated or chlorinated and of the acrylate, methacrylate or vinyl ether type. An embodiment of this kind also includes derivatives of the above reactive groups such as derivatives of vinyl ethers such as propynylethers.
Finally, the invention provides another method of continuously fabricating a graded index plastics optical fiber in which the refractive index is highest at the center and decreases progressively towards the periphery, said fiber having a plurality of layers of resins deposited concentrically, said method comprising
preparing at least two amorphous mixtures each comprising at least one primer, at least one at least partly fluorinated polymer P comprising from 10 to 1000 monomer units and a reactive diluting agent comprising at least two monomers M1 and M2 with different refractive indeces, each monomer being at least partly fluorinated and at least one of the two monomers M1 and/or M2 including at least two reactive groups adapted to react with a reactive group of one of the two monomers M1 or M2, the other of the two monomers M1 or M2 including at least one reactive group adapted to react with a reactive group of one of the two monomers M1 or M2, the proportion as a percentage by weight of said polymer P relative to the sum of the components of the mixture being practically constant for each mixture and the proportion as a percentage by weight of M1 relative to the sum of monomers of the reactive diluting agent varying significantly from one mixture to another,
extruding the first mixture with the highest refractive index to constitute the core,
depositing layer(s) with respective decreasing refractive indeces on said core,
interdiffusing each layer with the adjoining layer(s), and
crosslinking the polymer P and the monomers of the reactive diluting agent.
In an embodiment of the method of the invention the monomers of the reactive diluting agent are chosen from the family of alkyl xcex1 fluoro or xcex1,xcex2 difluoro acrylates or methacrylates, the term xe2x80x9calkylxe2x80x9d designating any at least partly chlorinated or fluorinated hydrocarbon group, and from at least partly chlorinated or fluorinated vinyl ethers.
In an embodiment of the method of the invention, the polymer P is prepared by radical polymerization and controlling the homogeneity of the molecular weights. For example, the polymer P is prepared from at least one monomer PM in the presence of a transfer agent and at least one thermal primer. In this case, the monomer PM is preferably selected from the alkyl xcex1 fluoro or xcex1, xcex2 difluoro (meth)acrylate family, the term xe2x80x9calkylxe2x80x9d designating any hydrocarbon group that is at least partly chlorinated or fluorinated, and at least partly fluorinated maleimides.
The method of the invention is preferably such that the layers are deposited successively. In a variant of the method of the invention at least partial intermediate crosslinking is performed between depositing each layer of mixture. In another variant of the method of the invention the layers are deposited successively, followed by a single crosslinking step.
The chemical behavior is preferably practically the same. Consequently, the variation in the composition of the mixture of monomers of the reactive diluting agent, mainly enabling refractive index to be modulated as a function of fluorine content, does not have any significant influence on the viscosity, reactivity, or thermal stability of the mixtures.
The plastics optical fiber of the invention has the advantage that it can be used in a waveband extending as far as the near infrared and has low attenuation (a few tens of dB/km) throughout the band.
Another advantage of the optical fiber of the invention is that it because it is crosslinked it can be used at temperatures which are high compared with the temperatures at which prior art plastics optical fibers can be used, e.g. up to at least 125xc2x0 C.
The fiber obtained in this way has a graded index type refractive index profile that is practically smooth and in which the variation of refractive index between the center and the periphery of the fiber is generally in the range from 0.01 to 0.03. The diameter of the fiber obtained in this way is generally in the range from 300 xcexcm to 1 mm.
The following examples illustrate the invention without limiting its scope.